1. Field of Invention
This invention relates to the field of optoelectronic devices and, more particularly, to organic light emitting devices. This invention further relates to methods of annealing organic light emitting devices.
2. Description of Related Art
Tang and Van Slyke reported efficient electroluminescence from a bilayer organic device in 1987. C. W. Tang and S. A. Van Slyke, xe2x80x9cOrganic Electroluminescent Diodes,xe2x80x9d Appl. Phys. Lett. 51, pp. 913-915, 1987. Since that time, organic light emitting devices (OLEDs) have attracted great attention because of their potential toward the fabrication of large-area displays. See, J. R. Sheats et al, xe2x80x9cOrganic Electroluminescent Devices,xe2x80x9d Science 273, pp. 884-888, 1996; J. Salbeck, xe2x80x9cElectroluminescence with Organic Compounds,xe2x80x9d Ber. Bunsenges. Phys. Chem. 100, pp. 1667-1677, 1996; and Z. Shen et al., xe2x80x9cThree-Color, Tunable, Organic Light-Emitting Devices,xe2x80x9d Science 276, pp. 2009-2011, 1997.
As described in the article by Sheats et al., the basic structure of an organic light emitting device comprises two electrodes and an organic material between the electrodes. Electrons are injected into the organic material from a low work function cathode, and holes are injected into the organic material from a high work function anode. The holes and electrons combine in the organic material to form a luminescent excited state.
Organic light emitting devices (OLEDs) represent a promising technology for various applications. These devices provide important advantages including versatile emission colors, high energy conversion efficiency and relatively low operating voltages. Organic light emitting devices can typically generate light at a brightness level of 100 cd/m2 at an operating voltage of less than 10 volts.
Although known organic light emitting devices can provide the above-stated advantages, there is still a need to further reduce the operating voltage of the devices, but without compromising their quantum efficiency. The quantum efficiency of an organic light emitting device can be defined as the number of emitted photons per electric charge driven into the device. A decrease in the operating voltage with no corresponding reduction in the quantum efficiency relates directly to an increase in the energy conversion efficiency and a decrease in the power consumption of a device including an organic light emitting device. A higher energy conversion efficiency, in turn, corresponds to lower power losses in the form of generated heat and, consequently, reduced device overheating.
This invention provides organic light emitting devices that can satisfy at least some of the above-described needs, as well as other needs. Exemplary embodiments of the organic light emitting devices according to this invention have reduced operating voltages. In addition, exemplary embodiments of the organic light emitting devices according to this invention have increased energy conversion efficiencies.
The organic light emitting devices according to this invention comprise as-fabricated organic light emitting devices that have been annealed. The annealed organic light emitting devices according to this invention comprise an anode, a cathode and a light emission region between the anode and cathode. The light emission region comprises an organic light emitting material.
Embodiments of the organic light emitting devices according to this invention can also comprise a hole transporting region and/or an electron transporting region adjacent to the light emission region.
The annealed organic light emitting devices according to this invention have improved performance characteristics as compared to prior to being annealed. Particularly, the annealed organic light emitting devices have reduced operating voltages, but they do not have reduced quantum efficiencies. Consequently, the annealed organic light emitting devices have increased energy conversion efficiencies.
Exemplary embodiments of the annealed organic light emitting devices according to this invention can emit light at an increased brightness level at a given driving current.
This invention also provides methods of annealing organic light emitting devices. Exemplary embodiments of the methods comprise heating an organic light emitting device at a temperature, and for a period of time, effective to improve the device performance characteristics of the device.
Exemplary embodiments of the methods of annealing organic light emitting devices according to this invention can decrease the operating voltage, and increase the energy conversion efficiency, of the organic light emitting devices, as compared to these same properties prior to the annealing.
In some exemplary embodiments of the methods of annealing organic light emitting devices according to this invention, the annealing results in the organic light emitting devices having increased brightness at a given driving current.
This invention further provides methods of annealing multiple organic light emitting devices, that may each have different properties, so as to make the properties of the different devices more uniform, i.e., more homogeneous. The as-annealed devices having more homogeneous properties can be used in the same device, or optionally in different devices, to provide more consistent device performance.